1、 Physical treatment
Physical treatment refers to the treatment method of changing the composition of wastewater by physical action. The physical treatment methods used in food industry wastewater treatment include screening, skimming, regulation, sedimentation, air flotation, centrifugal separation, filtration, microfiltration, etc. The first five processes are mostly used for pretreatment or primary treatment, and the last three are mainly used for advanced treatment.
Sieve filtration is the most widely used method in pretreatment. The main function is to separate coarse dispersed suspended solids from wastewater. The equipment used are grating and screen. The grid intercepts coarse suspended solids, which is used to protect the water pump and subsequent treatment equipment. The commonly used grid screens in food industry wastewater include fixed screen, rotating screen and vibrating screen. The most commonly used aperture of grid screen is 10-40 mesh.
Some food industry wastewater contains a large amount of grease, which must be removed before entering the biological treatment process, otherwise it will cause the blockage of pipelines, water pumps and some equipment, and also have a certain impact on the biological treatment process. In addition, oil removal and recovery has great economic value.
According to its physical state, the oil in wastewater can be divided into two categories: free floating and emulsified. Floating grease is usually removed from the oil separator. The removal rate of floating grease in the oil separator can reach more than 90%. If there is a regulating tank or sedimentation tank in the treatment process, the oil separation tank can share a unified structure with the regulating tank or primary sedimentation tank, which can save investment and land occupation. For small treatment system, oil-water separator can be set to skim oil.
For food industrial wastewater with large changes in water quality and quantity, a regulating tank is often set to regulate the water quality and quantity of wastewater. The regulating time is generally 6-24h, mostly about 6-12h. The capacity of the regulating tank is 15% - 50% of the daily wastewater treatment.
Precipitation is used to remove inorganic and organic solids from raw wastewater and separate solid and liquid phases in biological treatment process. Remove the inorganic solids in the original wastewater with a grit chamber; Remove the organic solids in the original wastewater with the primary sedimentation tank; The secondary sedimentation tank is used to separate the biological phase and liquid phase in the biological treatment process. The grit chamber is generally located behind the grid and screen. In order to remove the organic matter on the surface of inorganic solids in wastewater and avoid sedimentation of organic solids in wastewater in grit chamber, aeration grit chamber can be used. The use of primary sedimentation tank can reduce the load of subsequent process. The effect of removing suspended solids in the primary sedimentation tank is related to the processed raw materials and products. According to the flow direction in the tank, it is divided into horizontal flow sedimentation tank, vertical flow sedimentation tank and radial flow sedimentation tank. In order to improve the sedimentation efficiency of the sedimentation tank, the inclined plate (pipe) sedimentation tank can be formed by setting parallel inclined plates or inclined pipes in the sedimentation tank. The general precipitation time is 1.5-2.0h.
Air flotation is mainly used to remove emulsified oil, surfactant and other suspended solids from food industry wastewater. There are vacuum air floatation, pressurized dissolved air floatation and diffuser (plate) air floatation. When the wastewater enters the container air flotation tank, adding chemical coagulant or coagulant aid to the water can improve the removal rate of emulsified oil and colloidal suspended particles. According to the data, air flotation can remove more than 90% of grease and 40% - 80% of BOD5 and SS. HRT of air flotation tank is generally 30min.
For the advanced treatment of secondary treated effluent, the common method is filtration, which can adopt sand filter or composite filter media filter. According to the filtration rate, it is divided into slow sand filter and fast filter. Generally, the filtration rate of single-layer sand filter is 8 ~ 12m / h.
Chemical treatment refers to the application of chemical principles and chemical action to convert the pollutant components in wastewater into harmless substances, so that the wastewater can be purified. The chemical nature of pollutants is changed after chemical treatment, and the treatment process is always accompanied by chemical changes. Chemical treatment methods for food industry wastewater include neutralization, coagulation, electrolysis, redox, ion exchange, membrane separation, etc.
The chemical treatment process used in food industry wastewater treatment is mainly coagulation method. Coagulation method cannot be used alone, but must be combined with sedimentation, clarification or air flotation of physical treatment process to form coagulation sedimentation or coagulation air flotation. Coagulation sedimentation can be used as pretreatment of biological treatment or advanced treatment after biological treatment.
Coagulation sedimentation is an important method of water treatment. For some wastewater with small colloidal particles or colloidal solutions that are difficult or unable to settle, add chemical coagulants to form large particles that are easy to settle and remove them. The colloidal proteins and polysaccharides in wastewater can be removed by coagulating and sedimentation.
Commonly used reagents are: lime, ferrous sulfate, ferric chloride and aluminum sulfate. Lime is generally not used alone, but often used in combination with other agents. The optimal dosage and pH value should be determined by test.
Chemical redox is an effective method to transform pollutants in wastewater. The dissolved inorganic and organic substances in wastewater are oxidized or reduced to slightly toxic or non-toxic substances through chemical reaction, or transformed into a form that is easy to be separated from water, so as to achieve the purpose of treatment.
Ion exchange is mainly a method of using ion exchanger to exchange and remove harmful ions (including organic and inorganic) in water.
Biochemical treatment is one of the most important processes in organic wastewater treatment system. In the wastewater treatment of food industry, biological treatment process can be divided into aerobic process, anaerobic process, stabilization pond, land treatment and various combined processes formed by the combination of the above processes. Food wastewater is organic wastewater, and biological method is the main secondary treatment process, which aims to degrade COD and BOD5.
Aerobic biological treatment process can be divided into activated sludge process and membrane process according to the growth form of microorganisms. The former includes traditional activated sludge process, stage aeration process, biosorption process, complete mixing process, delayed aeration process, oxidation ditch, intermittent activated sludge process (SBR), etc. The latter includes biofilter, tower biofilter, biological rotary table, active biofilter, biological contact oxidation, aerobic fluidized bed, etc. Generally, aerobic treatment has good effect on low concentration wastewater.
Anaerobic biological treatment process is suitable for food industry wastewater. The main reason is that the wastewater contains high concentration organic matter that is easy to biodegrade and has no toxicity. In addition, the power consumption of anaerobic treatment is low, the generated biogas can be used as energy, the amount of surplus sludge generated is small, and the anaerobic treatment system is completely closed, which is conducive to improving environmental sanitation. It can operate seasonally or intermittently, and the sludge can be stored for a long time.